Bump django-debug-toolbar from 2.0 to 2.2.1 in /backend

Bumps [django-debug-toolbar](https://github.com/jazzband/django-debug-toolbar) from 2.0 to 2.2.1.
- [Release notes](https://github.com/jazzband/django-debug-toolbar/releases)
- [Changelog](https://github.com/jazzband/django-debug-toolbar/blob/2.2.1/docs/changes.rst)
- [Commits](django-commons/django-debug-toolbar@2.0...2.2.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -1,5 +1,5 @@
 Django==2.2.8
-django-debug-toolbar==2.0
+django-debug-toolbar==2.2.1
 psycopg2-binary>=2.8,<2.9
 dj-database-url==0.4.1
 ipython==5.3.0